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Measurements and calculations of reaction losses of medium-energy protons in NaI detectors Goulding, C. A.; Rogers, J. G. 1975

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MEASUREMENTS AND CALCULATIONS OF REACTION LOSSES OF MED IUM-ENERGY PROTONS IN Nal DETECTORSC.A. Goulding*Department of Physics, University of Manitobaand■J.G. Rogers*TRIUMF, University of Alberta^Present address: TRIUMFMESON FACILITY OF:UNIVERSITY OF ALBERTA SIMON FRASER UNIVERSITY UNIVERSITY OF VICTORIA UNIVERSITY OF BRITISH COLUMBIA TRI-75-3TRI-75-3TRIUMFMEASUREMENTS AND CALCULATIONS OF REACTION LOSSES OF MED IUM-ENERGY PROTONS IN Nal DETECTORSC.A. Goulding*Department of Physics, University of ManitobaandJ.G. Rogers*TRIUMF, University of Alberta-'Present address: TRIUMFPostal address:TRIUMFUniversity of British Columbia Vancouver, B.C.Canada V6T 1W5 December 1975ABSTRACTReaction losses of protons in Nal have been directly measured at 86, 133. and 146 MeV. Calculations of the expected tail-to-peak ratios were made over the range 40 to 240 MeV using recent total reaction cross-section data for sodium and iodine. The calculated ratios agree well with the data and are slightly larger than the previously calcu­lated values of Measday and Richard-Serre.1. INTRODUCTIONAn extensive series of elastic and quasi-elastic cross-section measurements are being made at TRIUMF by several experimental groups. 1 These measurements all involve detect ion of protons in one or more detector telescopes, each composed of a multi-wire proportional counter (MWPC), one or more thin plastic scintillators, and a sodium iodide (Nal) total energy detector. 2 Because these telescopes will be used to measure absolute cross-sections, it is necessary to accurately know the absolute efficiency of the telescopes for detection of protons (and other particles) in the medium-energy range. The efficiency of the telescopes has been measured for protons over the range of proton energies from 85 to 450 MeV . 2*3 The Nal detectors are thick enough to stop protons with energies up to 150 MeV, so degraders are not needed as part of the telescopes at the lower energies. For these cases the desired efficiency is determined only by the reaction losses in the Nal total energy detector. These losses may be calculated if the total reaction cross-sections (or) for sodium and iodine are known. 11 This report summarizes the method used to extract the Nal reaction losses from the efficiency data at three energies below 150 MeV. We compare these measured losses with results of a calculation based on the best currently available reaction cross-section data. We have obtained rather good agree­ment between the measured and calculated losses, which allows us to use the calculations with confidence at energies where no accurate measurements exist. .2. EFFICIENCY MEASUREMENT TECHNIQUE AND DATA ANALYSISThe efficiency measurement consisted of detecting proton-proton coincidences from a CH2 target in two identical detector telescopes. Each telescope contained a cylindrical Nal detector 5 in. in diameter and 3 in. thick viewed by a 5 in. photomultiplier. By selecting the angles to be those appropriate to proton-proton elastic scattering, the scattering from the carbon in the CH2 was reduced to a small percentage of the hydrogen scattering. The energy incident on the Nal detector was calculated from the known proton bombarding energy and scattering angle, taking into account energy losses in the MWPC and plastic scintillators. The energy at the Nal detector could be conveniently varied by changing the scattering angle.- 2 -With a proton bombarding energy of 250 MeV we were able to extract the Nal reaction losses at energies of 86, 133, and 146 MeV. The selection of events to be acquired was done by requiring only a left-right coincidence between the first detectors in the two telescopes. All events satisfying this requirement were recorded on magnetic tape by a computer. The data on tape are analyzed by reading the tapes back into the computer and forming selected events into histograms. The selection of events to be formed into histograms is made by requiring that various of the recorded parameters for each event lie between specified limits. The objective of the selection process is to choose from the events recorded on tape a group which corre­sponds to a monoenergetic uniform flux of protons incident on the centre of the circular face of the Nal crystal.All the data recorded on tape do not correspond to such a uniform flux because of the small number of constraints applied by the electronics when the data were acquired. In the acquisition of the data the two tele­scopes were treated identically. In the later analysis of the data on tape the proton detected in one telescope is used as a tagging signal to define a monoenergetic flux at the other telescope. By interchanging the roles of the two telescopes in the analysis, a measurement at one angle setting can be used to obtain the Nal reaction losses at two different energies.A typical pulse height spectrum in the Nal from such a selected group of events is shown in Fig. 1. The quantity which conveniently characterizes the reaction losses in the Nal detector is the ratio of the number of counts in the 'tail' region to the number of counts in the full-energy peak. This ratio is a measure of the probability that a proton of a particular energy (146 MeV in the case of Fig. 1) will suffer an inelastic nuclear reaction before being stopped by the normal dE/dx process. To measure this ta i1-to-peak ratio accurately it is important that the 'incident beam' defined by the apparatus and analysis does not itself contain a low-energy tail. In addition, it is important to avoid any geometrical edge effects in the Nal because this type of loss varies from detector to detector and is more difficult to calculate accurately than is the bulk reaction loss.To avoid geometrical edge effects the protons incident on the Nal are collimated in the analysis by placing limits on the co-ordinates from the MWPC detector. The accepted protons are required to pass through a- 3 -square aperture of size 6.8 cm * 6.8 cm centred on the 5 in. (12.7 cm) cir­cular face of the Nal crystal. In addition to the collimation requirement in the Nal used for the loss measurement, it is required that events have a detected energy in the other (tagging) Nal which corresponds to elastic hydrogen scattering. These two requirements reduce the incident low-energy contamination from carbon to a fraction of one per cent of the tail from the Nal reactions. This background is then subtracted utilizing data from a similar measurement and analysis with a pure carbon target. A correction is also made for protons which suffer a reaction in one of the thin plastic coun­ters before reaching the Nal. Using the tables of Ref. 4 for NE102 reaction losses we make corrections ranging from 1.15% at 86 MeV to 0.66% at 146 MeV.The separation of the measured spectra into tail and peak regions is made at a point 7 MeV below the centroid of the ful1-energy peak, as indi­cated by the vertical arrow in Fig. 1. This choice of where to make the separation between tail and peak regions is somewhat arbitrary. Because of the small reaction Q-value and the finite detection resolution in the Nal it is not possible to cleanly separate the data into elastic and inelastic regions. However, the effect of changing the position of the cut by a few MeV is small. The tail-to-peak ratios for the three energies measured are given in Table I. The last column in Table I is the absolute amount by which the tail-to-peak ratio changes when the separation point between tail and peak is changed. These numbers may be used to adapt the tabulated value of reaction loss to an energy cut other than the one we used. In addition to the kinematically-determined energy of the protons passing through the centre of the MWPC aperture, the first column of Table I also gives the approximate energy spread due to kinematic broadening and finite target thickness.Table I. Measured Nal reaction lossesThe energy spread quoted in the first column is that due to kinematic broadening and finite target thickness. The uncer­tainty quoted in the second column is that due to statistics.EP(MeV)Tai1/Peak Ratio (%)A (Tail/Peak)/AE (%/MeV)86 ± 4 8.0 ± 0.2 0.14133 ± 4 17.4 ± 0.3 0.17146 ± 4 20.0 ± 0.3 0.31- 4 -3- CALCULATION OF THE EXPECTED TAIL-TO-PEAK RATIOThe method of Measday and Richard-Serre4 was used to calculate the expected tail-to-peak ratio for Nal and several other materials. The input to the calculation for Nal are'the total reaction cross-sections for sodium and iodine as a function of proton energy, shown in Fig. 2. The curves shown in Fig. 2 are based on recent measurements of reaction cross-sections. 5 For comparison, the older values of or used by Measday and Richard-Serre are also shown.The ambiguity of how to separate the reactions into peak and tail regions also makes a small uncertainty in the calculation of the expected tail-to-peak ratio. The calculation performed in Ref. 4 and in this work is an integration of the inelastic cross-section as a function of energy over the range of the proton as it slows down in the material. It is assumed that all inelastic events occurring as the proton slows down contribute to the tail region of the spectrum. To approximately account for the production of ‘slightly inelastic1 protons which cannot be resolved from the ful1-energy peak, the integration procedure is stopped when the proton has been degraded to some small energy in the material. In our case we chose this cut-off energy to be 5 MeV.Fig. 3 shows the calculated tail-to-peak ratios over the energy rangeEp = 40-240 MeV. Table II contains the same information in tabular form.**■ COMPARISON OF CALCULATED AND MEASURED TAIL-TO-PEAK RATIOSFig. 4 shows our data along with other measured data on Nal reactionlosses in the energy range we measured. The solid curve is the calculationdescribed in Sec. 3 of this report. The calculation of Measday and Richard- Serre is shown as a dashed curve. Our calculation using the revised values of M E fits the Nal reaction loss data slightly better than the earlier calculat ion.ACKNOWLEDGEMENTSWe are pleased to acknowledge the contributions of J.M. Cameron,P. Kitching, D.F. Measday, and W.T.H. van Oers in reading and commenting on the manuscript.- 5 -Table II. Tail-to-peak ratio protons in Nal as a function of proton energyEnergy(MeV)Ta i1/Peak(%)Energy(MeV)Ta i1/Peak(%)40 2.11 145 19.7845 2.56 150 21 .0050 3.11 155 22.2655 3.70 160 23.5560 4.32 165 24.8865 4.96 170 26.2570 5.64 175 27.6575 6.36 180 29.088o 7.12 185 30.5685 7.90 190 32.0890 8.71 195 33.6395 9.55 200 35.24100 10.42 205 36.89105 1 1 .32 210 38.56110 12.27 215 40.28115 13.24 220 42.08120 14.22 225 43.89125 15.27 230 45.76130 16.35 235 47.68135 17.46 240 49 • 66140 18.60- 6 -REFERENCES AND NOTES1. TRIUMF experiments which propose to use such detector telescopes includeElastic Scattering (proposals 1* and 2*), Quasi-elastic Scattering(proposals 16 and 58) and (p,2p) Reactions (proposal 59).2. The MWPC detectors were designed and built by E.B. Cairns at the Uni­versity of Alberta. The telescope assemblies were designed by A.W. Stetz at the University of Alberta. A partial description of the telescopes is contained in:A.W. Stetz, TRIUMF design note TRI-DNA-75-* (unpublished).3. J.M. Cameron, C.A. Goulding, P. Kitching, R. McCamis, C.A. Miller,G.A. Moss, J.G. Rogers, G. Roy, A.W. Stetz, W.T.H. van Oers, University of Alberta TRIUMF Progress Report 1975 (unpublished).*. D.F. Measday and C. Richard-Serre, CERN report 69“17 (1969).5. W.F. McGill, R.F. Carl son, T.H. Short, J.M. Cameron, J.R. Richardson,I. Slaus, W.T.H. van Oers, J.W. Verba, D.J. Margaziotis and P. Doherty, Phys. Rev. CIO, 2237 (197*0;J.J.H. Menet, E.E. Gross, J.J. Malanify and A. Zucker, Phys. Rev. C*, 111* (1971);P. Kirkby and W.T. Link, Can J. Phys. **, 18*7 (1966);P.U. Renburg, D.F. Measday, M. Pepin, P. Schwaller, B. Favier and C. Richard-Serre, Nucl. Phys. A183, 81 (1972).10000- 7 -o  —13NNVH0/SINOOOFig. 1. A typical Nal energy spectrum. The arrow indicates the division point between tailand peak regions of the spectrum. The tail is taken to be the region to the left ofthe arrow and the peak to the right.- 8 -■-3CXI<35ft.Total reaction cross-sections used in the calculations. The solid curves are the cross- sections used in this work to compute the reaction loss in Nal. They are based on the measurements of Ref. 5. The +rs are cross-sections used by Measday and Richard-Serre ^at selected values of energy.TAIL / PEAK (%)- 9 -Ep (MeV)Fig. 3. Calculated Nal reaction losses as a function of incident energy.The solid curve is the calculation described in Sec. 3 of thisreport. The dashed curve is the calculation of Ref. 4.TAIL/PEAK (%)- 10 -Nal REACTION LO SSE S• Present measurement □ Palmieri and Wolfe from ref. 4Present calculation Measday and Richard-Serre calculation80 90  100 110 120 130 140 150Ep (MeV)Fig. 4. Measured and calculated Nal reaction losses as a function of incident energy. The solid and dashed curves are the same as in Fig. 3.BEST-PRINTER CO LTD. VANCOUVER. B.C.


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